In my last column, I promised you'd learn to love point spread function (PSF), particularly when using it to identify your patients' lower- and higher-order aberrations (HOAs). In this column, I'll describe how you can do this with "the poor man's aberrometer."

HEAR THE MAGIC WORDS

We describe PSF as a point of light on a dark background. Thus, negative PSF would be a dark spot on a light background, which pretty much describes a letter on the good old Snellen chart. Since PSF distortions result from lower- and higher-order aberrations in an optical system, we're actually measuring our patients' aberrations when we have them read the Snellen chart. This is why I refer to the Snellen chart as the poor man's aberrometer.

Whether a patient can read the 20/30 line, the 20/20 line or even the 20/10 line, when he says those magic words, "But it's not clear," he's telling you he has clinically observable HOAs. We've come to understand that clinically observable HOAs are usually about 0.4 root mean square (RMS) units or greater. Thus, those magic words have given you a relative quantification of your patient's HOAs. Lo and behold, that patient of yours isn't crazy after all!

ASK A SIMPLE QUESTION

Now that you know your patient has HOAs, how do you determine what type he has? If you ask him, he'll tell you.

Your next question should be, "Can you tell me more about what you're seeing?" By giving your patient an opportunity to describe what he's seeing, you usually can identify the HOAs affecting his visual system.

►Coma. Coma is basically plus and minus power coming through the pupil simultaneously in a vertical, horizontal or oblique orientation. With coma, patients will describe an overlapping or secondary image or even a double image. Sometimes, patients may even tell you the direction in which the distortion is occurring.

►Trefoil. Another third-order aberration, trefoil is three "wings" of variable power in the pupil. Patients usually describe this as "tails of light" coming off a letter. Again, based on its orientation, they may be able to define the direction as well.

►Fourth-order Spherical Aberration. This is a symmetrical distribution of power within the pupil. It's often described by the patient as haze, halos, starbursts or glare around the letter.

FIND THE SOURCE

When patients who've just read the 20/20 line tell us they're not seeing clearly, our first reaction may be to think they're crazy. But the lowest "readable" line on the Snellen chart doesn't necessarily mark a refractive end point.

We now know up to 20% of vision is affected by HOAs, and that's what patients are telling us when they say they're not seeing clearly, even at 20/20. By going a step further and asking patients exactly what they're seeing, we often can identify the source of their vision problem. nOD

Dr. Catania is associated with Nicolitz Eye Consultants in Jacksonville, Fla. He does clinical research on developing eyecare technologies; consults for ophthalmic companies and professional journals; holds academic ranks at numerous educational institutions; and writes and lectures worldwide. You can reach him at
lcatania@bellsouth.net.